Wireless mobile communications devices, such as a mobile handset, are widely popular. Such wireless devices often have a speaker for producing audio sounds. For example, the speaker can reproduce a caller's voice, generate ringing tone sounds, play music or facilitate hands-free device operation.
Wireless mobile communications devices also may have a silent alert mechanism, such as a mechanical vibrator or flashing light. The silent alert quietly notifies the user of an event associated with the wireless mobile communication device. In this manner, the user is notified of the event while others situated nearby are not disturbed. For example, a mechanical vibrator silent alert creates a mechanical vibration that causes the wireless mobile communication device's enclosure to vibrate. A user in physical contact with the enclosure will sense the vibrations and thereby become notified of the event. Such a silent alert is useful when the user is in an environment in which an audio alert is prohibited. For example, a user may want a silent alert to notify the user of an incoming phone call while the user is inside a movie theater.
A wireless mobile communication device often has both a speaker and a mechanical vibrator alert. The speaker and the vibrator operate independently of each other and provide different functionality for the mobile wireless communication device. The speaker operates to generate audio signals for the user of the wireless mobile communication device, while the vibrator generates a mechanical vibration to alert the user of an event. Although the speaker may generated audio signals while the vibrator generates mechanical vibrations, the speaker and vibrator usually operate at different times such that only one of them is active at a given time.
In an effort to reduce costs and part count, the wireless mobile communication device may include a speaker that can generate a mechanical vibration for alerting the user. Such a speaker has both an acoustical resonant frequency and a mechanical vibration resonant frequency. The acoustical resonant frequency is within the speaker's audible or acoustical frequency range. The mechanical vibration resonant frequency is usually a low frequency below the speaker's audible frequency range. The speaker contains a moving mass that vibrates in response to a drive signal having the speaker's mechanical vibration resonant frequency. Such a speaker can be driven near the speaker's mechanical vibration resonant frequency to cause mechanical vibration in the speaker. The speaker is coupled to the wireless mobile communication device to facilitate the transfer of the vibrations from the speaker to the enclosure of the wireless mobile communication device. In this manner, the speaker operates as a mechanical vibrator silent alert.
The vibration force of the speaker is dependent upon the difference between the drive signal's frequency and the speaker's mechanical vibration resonant frequency. The vibration force in the speaker is greatest when the drive signal's frequency is equal to the speaker's mechanical vibration resonant frequency. The vibration force in the speaker decreases as the difference between the drive signal's frequency and the mechanical vibration resonant frequency increases. For all practical purposes, the speaker vibrates with sufficient vibration force to cause mechanical vibration in a narrow band of frequencies surrounding the mechanical vibration resonant frequency. This narrow band of frequencies is the excitation frequency range of the speaker. If a speaker is driven in the speaker's excitation frequency range, the speaker will mechanically vibrate to some degree. The speaker will vibrate with the largest amplitude at the center of the speaker's excitation frequency range, which is the speaker's mechanical vibration resonant frequency.
The speaker's mechanical vibration resonant frequency varies from speaker-to-speaker. A selected speaker from a group of speakers typically mechanically vibrates at a slightly different frequency than other speakers in the group. The frequency range encompassing all the speakers' mechanical vibration resonant frequencies in the group is the mechanical vibration resonant frequency range for the given group. This variation in mechanical vibration resonant frequencies makes it difficult to manufacture wireless mobile communication devices with the same drive signal source. If the drive signal source has only a single frequency, some of the speakers in the group will not vibrate sufficiently at that frequency. This presents significant difficulty in selecting a speaker for the wireless mobile communication device and therefore in manufacturing the wireless mobile communication device with the speaker.
One method that has been used to overcome this problem is driving the speaker with a composite signal having multiple frequencies. The composite signal is constructed by adding together multiple signals having different frequencies within the speakers' mechanical vibration resonant frequency range. Driving the selected speaker from the group of speakers with the composite signal increases the probability that one of the drive signal's constituent frequencies is within the excitation frequency range of the selected speaker. In this manner, the probability of mechanically vibrating each select speaker by driving the speaker with the composite signal increases significantly.
However, the composite signal typically has one or more beat frequencies. The beat frequencies are caused by the constructive and destructive interference of the composite signal's multiple constituent frequencies. The beat frequencies effectively drive the speaker at additional undesirable frequencies outside the speaker's excitation frequency range.